In prior art, the current limit of the LDO/Amplfier or the like was reduced at startup in order to reduce the startup current. It was restored to normal current limit once the output voltage reached within 90% of its regulated voltage. If the output capacitor was relatively large this would result in a sudden increase in the inrush current when the normal current limit was restored. This could result in an overshoot at the output. The inrush current would vary a lot with process, temperature and supply.
The slope of the startup current was not controlled. A very sharp edge of inrush current would act as shock wave for decoupling capacitor and interfere with audio signal on a handheld.
In case a handheld device is getting charged with a current limited supply (as e.g. an USB with 100 mA current) a large inrush current at startup may discharge the decoupling capacitors on the supply and result in a system shutdown.
In case an output smoothing capacitor is relative large this would result in a sudden increase of the inrush current when the normal current limit is restored. This could result in an overshoot at the output.
The slope of the startup current is not controlled in prior art. A very sharp edge of inrush current would act like a shock wave for a decoupling capacitor and would interfere e.g. with audio signals on a handheld device
In case a handheld device is getting charged with a current limited supply, e.g. an USB with 100 mA current limit, a large inrush current at startup may discharge the decoupling capacitors on the supply and result in a system shutdown.
The scenario showed in FIG. 11 prior art illustrates a non-limiting example of a possible application of the present disclosure.
The system of FIG. 11 prior art shows a mains charger powering a power management integrated circuit (PMIC) and charging a battery. The switch shown can be used to charge the battery when a charger is attached or can be used in absence of charger to power the PMIC from battery. The PMIC may comprise for example several low drop-out (LDO) regulators and some buck DC-to DC converters.
In case the charger circuit is both charging the battery and powering the PMIC. The maximum allowable current from the charger may be I1.
Under no condition should the sum of currents I2+I3 get higher than current I1, if that happens the charger circuit will be overloaded and the output voltage from the charger will fall causing the PMIC to reboot.
When an LDO or a buck converter is enabled the output decoupling capacitors (not shown) will have to be charged. The maximum current during startup would be limited by a current limit of the buck converter or the LDO. If this current limit is higher than the difference I1−I3, which may be well possible, the system may shutdown and goes into a loop of starting and shutting down.
The startup current for the sub-blocks of PMIC has to be regulated in order to avoid a situation like this. The current at startup must also be independent of supply, process and temperature.
Charger systems have an output impedance, bandwidth and maximum current capability. As the charger system is external to PMIC these parameters may vary a lot. When any of the sub-blocks in the PMIC is enabled during charging process the current at startup would come from supply decoupling capacitors at the input of PMIC (not shown). This would require large decoupling capacitors which would occupy large area on the printed circuit board (PCB) which is very expensive for a handheld device.
The amount of decoupling capacitors would be reduced if the startup current could be well regulated and the time taken to reach the maximum regulated current at startup be controlled.
It is a challenge for designers of low drop-out (LDO) converters, amplifiers, DC-DC converters, or the like to achieve a controlled linear method of limiting a constant current during startup independent of the size of a load capacitor with reduced dependence on process, supply and temperature to avoid any harmonics created in the audio band during startup, and achieve a clean startup when getting charged with a current limited supply.